Ultrafine particle steel can extremely enhance the strength without adding alloy elements, and simultaneously reduces the ductility and brittleness transition temperature extremely, and hence the present inventors have been studying about measures for realizing this ultrafine particle steel industrially, and have already disclosed inventions, warm multipass rolling method (document 1) and multidirectional processing method (document 2).
On the other hand, various methods have been proposed so far for manufacturing ultrafine particle steel, but nothing has been known about the method of controlling the grain size quantitatively.
For example, Fujioka et al. (document 3) proposed a manufacturing method of high tension steel characterized in using a billet comprising C, 0.03 to 0.45 weight (wt.) %, Si: 0.01 to 0.50%, Mn: 0.02 to 5.0%, Al: 0.001 to 0.1%, and balance of Fe and inevitable impurities, processing it by one pass or two or more consecutive passes with interval of each pass of within 20 seconds, at temperature of 500 to 700° C., strain speed of 0.1 to 20/sec, and total strain amount of 0.8 to 5.0, and then cooling slowly.
They also proposed a manufacturing method of high tension steel of fine crystal particles characterized in using a billet comprising, by wt. %, C, 0.03 to 0.9, Si: 0.01 to 1.0, Mn: 0.01 to 5.0, Al: 0.001 to 0.5, N, 0.001 to 0.1, and also at least one of Nb: 0.003 to 0.5 and Ti: 0.003 to 0.5, and balance of Fe and inevitable impurities, and satisfying the relation of C+(12/14)N≧(12+48)Ti(12/487)Nb+0.03, casting it or heating it and cooling it once into the temperature range of 500° C. to room temperature after rolling or without rolling, heating,
rolling in warm process at 700 to 550° C. by processing it in one pass or two or more consecutive passes with pass interval of within 10 seconds with draft per pass of 20% or more, at strain speed of 1 to 200 sec, and total strain amount of 0.8 to 5, and then cooling slowly (document 4).
In these two methods, however, nothing is taught about control method of grain size. In these methods, moreover, the pass duration is limited and the strain speed is also limited, and it is considered difficult to apply industrially.
In this background, the inventors have further promoted studies, and found that it is important to control the cumulative strain in multipass rolling, processing temperature, strain speed, and pass duration comprehensively, not individually, in order to form an ultrafine crystal structure. As a result, it is known that the grain size depends on parameter Z of processing temperature and strain speed expressed in formula (1); and the present inventors have proposed a new control method of grain size by clarifying the relation of Z and grain size through one-pass rolling experiment (prior application 1).
                    Z        =                  log          ⁡                      [                                          ɛ                t                            ⁢                              exp                ⁡                                  (                                      Q                                          8.31                      ⁢                                              (                                                  T                          +                          273                                                )                                                                              )                                                      ]                                              (        1        )                            ε: strain        t: duration from start till end of rolling (s)        T: rolling temperature (° C., or average of rolling temperature of each pass in the case of multipass rolling)        Q: 254,000 if mother phase of texture just before rolling is ferrite, bainite, martensite, or pearlite; 300,000 if mother phase is austenite.        
In this method, to manufacture ultrafine ferrite steel with crystal size of 1 microns or less, it is found that the rolling condition parameter Z in formula (1) should be 11 or more (in the case the texture before rolling is ferrite, bainite, martensite, pearlite or the like, that is, the iron crystal structure is bcc), and that the strain speed can be defined by the value of the total strain ε being divided by the time t from start till end of rolling, and hence ultrafine crystal particles of 1 micron or less can be obtained in the condition of strain e=3.0 and total rolling time t=300 s, that is, strain speed=0.01/s, and this newly proposed method can be applied in a wide range.
According to this method, the grain size can be controlled without limitation in pass interval or strain speed.
However, in the subsequent process of researches by the present inventors, new problems have been also unveiled, that is, the actual rolling is continuous multipass process, and when the parameter Z is 11 or more, the rolling temperature corresponds to the warm processing temperature region (350 to 800° C.), and the deformation resistance of steel is large and the processing heat generation of material is large in this case, and the material temperature may rise several hundred degrees during continuous rolling, there by resulting in Z<11, and ultrafine structure of 1 micron may not be formed.
It has been hence demanded to develop a method capable of controlling the grain size stably even in such continuous multipass rolling.
Document 1: Japanese Patent Application Laid-Open No. 2000-309850
Document 2: Japanese Patent Application Laid-Open No. 2001-240912
Document 3: Japanese Patent Application Laid-Open No. 9-279233
Document 4: Japanese Patent Application Laid-Open No. 2000-104115
Prior application 1: Japanese Patent Application No. 2002-54670
The present invention is applied in the light of the above background, and presents a new control rolling method in consideration of processing heat generation, as a method capable of applying the new method of controlling the parameter Z according to the proposal of the inventors in continuous rolling process, stably manufacturing ultrafine crystal steel of 3 microns to 1 micron or less without any limitation in pass internal or strain speed.